The physical origin of electric polarizations in two dimensional ferroelectric monolayer materials

Over the past decade, a new class of ferroelectric materials with atomic-level thickness, particularly monolayer materials, has been predicted theoretically and confirmed experimentally. These two-dimensional ferroelectric materials, especially those exhibiting finite out-of-plane (OOP) polarizations, have garnered significant attention in both condensed matter physics and materials science. On one hand, they offer promising avenues for the miniaturization of ferroelectric devices. On the other hand, they reveal novel physical mechanisms that go beyond those found in conventional bulk ferroelectrics, enabling the emergence of OOP polarization under depolarization fields. Recent studies have identified various mechanisms capable of generating OOP polarization in monolayers, a phenomenon previously considered unlikely in traditional bulk materials like ferroelectric perovskites. This review article highlights the recent advancements in understanding two-dimensional ferroelectricity in monolayer candidates. We focus primarily on the exploration of these unique mechanisms, as investigated and rationalized in recent years. Furthermore, we discuss the promising prospects in this emerging field of ferroelectricity and the bright future of two-dimensional monolayer materials.